Today, there are many radio and cellular access technologies and standards such as GSM/GPRS, WCDMA/HSPA, CDMA-based technologies, WiFi, WiMAX, and LTE, to name a few. These technologies and standards have been developed during the last few decades, and it can be expected that the development will continue. The technology in this application primarily focuses on the high speed packet access (HSPA)-evolution built on the WCDMA radio access also called UTRAN, and LTE, which is based on OFDM and SC-FDMA, also recognized as the Long Term Evolution of UTRAN, or E-UTRAN.
Multi-carrier or carrier aggregation may be used to enhance peak-rates within a radio access technology (RAT). For example, it is possible to use multiple 5 MHz carriers in a HSPA-based RAT to enhance the peak-rate within the HSPA network. Similarly, there is a plan for LTE Release 10 to facilitate aggregation of multiple LTE carriers, e.g., aggregation of multiple 20 MHz carriers. In forthcoming evolutions of cellular system standards like the Third Generation Partnership Project's (3GPP's) Long Term Evolution (“LTE”) the maximum data rate is sure to be higher than in existing systems. Higher data rates typically require larger system radio spectrum bandwidths. For the International Mobile Telecommunications-Advanced (“IMT-Advanced”) system (i.e., the fourth generation mobile communication systems) bandwidths up to 100 MHz are being discussed. A problem being faced is that the radio spectrum is a limited resource that has to be shared by many operators and systems; this makes it very complicated to find 100 MHz of free contiguous spectrum that can be allocated.
One method of overcoming this problem is aggregating contiguous and non-contiguous spectrum. FIG. 1 below shows an aggregation of two 20 MHz bands 201, 203 and one 10 MHz band 205. The 20 MHz band 203 and the 10 MHz band 205 are contiguous, whereas the 20 MHz band 201 is separated from the 20 MHz and 10 MHz bands 203, 205 by some amount of spectrum 207. The benefit of such a solution is that it becomes possible to generate sufficiently large bandwidths (e.g., 50 MHz in the example of FIG. 1) for supporting data rates up to (and above) 1 Gb/s, which is a throughput requirement for a fourth generation (“4G” or IMT-advanced) system. The ability to utilize an aggregation of noncontiguous as well as contiguous bands of the radiofrequency spectrum makes it possible for communication system operators to adapt which parts of the radio spectrum will be used based on present circumstances and geographical position
Each carrier in multi-carrier or carrier aggregation system is generally termed as a component carrier (CC) or sometimes is also referred to a “cell” (not to be confused with a geographical cell area). A component carrier (CC) is an individual carrier in a multi-carrier system. Carrier aggregation (CA) is also called “multi-carrier system,” “multi-carrier operation,” and “multi-carrier” transmission and/or reception. CA is used for transmission of both signaling and data in the uplink and downlink directions. One of the CCs is the primary carrier or anchor carrier, and the remaining CCs are called secondary or supplementary carriers. Generally, the primary or anchor CC carries the important UE-specific signaling. The primary CC exists in both uplink and downlink directions. The network may assign different primary carriers to different UEs operating in the same sector or cell.
The CCs belonging to the CA may belong to the same frequency band (intra-band CA), to different frequency bands (inter-band CA), or a combination thereof (e.g. 2 CCs in band A and 1 CC in band B). Furthermore, the CCs in intra-band CA may be adjacent or non-adjacent in the frequency domain (intra-band non-adjacent CA). A hybrid CA comprising intra-band adjacent, intra-band non-adjacent, and inter-band is also possible.
For an operator with a certain bandwidth that must deploy two or more radio access technologies (RATs), e.g., HSPA and LTE, if the bandwidth offered in the specific or individual RAT technology is limited to part of the given bandwidth, these carrier aggregation approaches within a RAT cannot fully utilize the whole operator bandwidth. To solve this problem, simultaneous use of multiple radio access technologies (RATs) may be used, i.e., multi-RAT carrier aggregation. Multi-RAT carrier aggregation is also termed as multi-RAT multi-carrier, inter-RAT CA, inter-RAT multi-carrier etc. For consistency, the term multi-RAT carrier aggregation (CA) is used. A multi-RAT CA scenario may include adjacent carriers and/or non-adjacent carriers. Non-adjacent carriers may or may not belong to the same frequency band which means that multi-RAT CA may be intra-band (all RATs in same band) or inter-band (at least 2 RATs/carriers in different bands). Non-limiting examples of other multi-RAT CA scenarios are: 1) LTE and CDMA2000, 2) LTE and GSM, 3) LTE, HSPA, and GSM, etc.
FIG. 2 shows an example multi-RAT cellular communication system with an LTE-based RAT serving cell area 105 and an HSPA-based RAT servicing cell area 109. The LTE base station is an evolved Node B (eNodeB or eNB) 101 that serves a user equipment (UE) 103 located within the serving node's geographical service cell 105. The HSPA base station is a Node B 107 that serves UE 103 located within the serving node's geographical service cell 109. Communication is bidirectional between each base station and the UE 103. Communications from each base station to the UE 103 are referred to as taking place in a downlink (DL) direction, and communications from the UE 103 to the eNB 101 are referred to as taking place in an uplink (UL) direction.
FIG. 3 shows an example of multi-RAT carrier aggregation of two carriers (HSPA & LTE) with HSPA acting as the “master” system. In this example, the first radio access technology (RAT) corresponds to the HSPA carrier and the second RAT to the LTE carrier. MAC-i/is PDUs (protocol data units) are generated at the UE and further distributed over the two different access technologies. The MAC-i/is PDUs are separately transmitted by the UE over the physical layers (L1) of the two different RATs using HSPA and LTE carriers. The Node B receives and demodulates the two carriers, and respective Hybrid ARQ (HARQ) entities at the NodeB operate separately on each carrier. Once successful decoding occurs on either or both carriers, the MAC-i/is PDUs are forwarded to the MAC-i layer of the “master” (HSPA) system where the PDUs from the two different carriers are aggregated. The aggregated MAC-i/is PDUs are then transmitted from the NodeB to the RNC over the Tub interface. In the RNC, the MAC-is PDUs may then be further processed for delivery to higher layers like the RLC protocol, which supports, among other things, selective repeat ARQ and encryption of user-plane data.
In multi-RAT carrier aggregation where HSPA and LTE are used simultaneously by both the NodeB and the UE, it is desirable for the UE to be able to simultaneously receive data from LTE and HSPA carriers and to simultaneously transmit data over LTE and HSPA carriers. The LTE and HSPA carriers should also be time-aligned to reduce UE complexity, processing, and cost.
Control signaling should also be considered for multi-RAT carrier aggregation. One control channel (i.e., HS-DPCCH or PUCCH) may be used to send feedback information (e.g., ACK/NACK, CQI, etc.) related to both RAT systems (HSPA and LTE). Using one control channel has a number of advantages including reducing signaling overheads, eliminating control channel bottleneck (a larger number of users results in delay in scheduling with a single control channel, but with two control channels in an overload situation on just one of them, it is possible that a UE cannot be scheduled on that overloaded control channel), reducing UE power backoff (cubic metric) in HSPA (in HSPA, the HS-DPCCH channel requires the UE to apply larger power backoff as compared to the PUCCH in LTE so using the PUCCH leads to less loss of the uplink coverage due to lower UE power backoff), and reducing BS complexity because only one control channel needs to be demodulated.
There are several characteristics of a control channel to consider. For example, the control channel is typically sent on the carriers of the primary RAT system. The uplink (UL) control channel is typically sent by a UE a fixed offset time period after downlink (DL) reception. Given that the same UL control channel typically carries feedback information for both RATs, multiple DL data channel transmissions should be received by the UE at about the same time. Otherwise, the UE has less time for processing the UL control channel because the offset time is fixed from the last received DL data channel transmission. As a result, the UE has to store information for a longer time which undesirably increases UE memory requirements.
One important aspect of cellular communication is to keep the uplink and downlink signals synchronized with one another between the base station and the user equipment. The UE transmit timing for uplink channels (e.g., RACH, UL data channel, UL control channel etc.) may be regularly adjusted for various reasons, e.g., for maintaining the UL orthogonality of uplink signals received from multiple UEs at a base station, for maintaining a fixed timing relation between the UL and DL timing, to compensate for the propagation delay between the UE and the base station, etc. In order to maintain orthogonality between multiple UE signals in the uplink direction, a timing adjustment parameter value typically needs to be sent from the network node to the user equipment.
For UE transmit timing in LTE, the eNode B adjusts the UE transmit timing by sending a timing advance command (an example of a timing adjustment (TA) parameter value) to the UE and a cell-specific reference signal (CRS) (an example of a timing reference signal broadcast by a base station). Each timing advance command tells the receiving user equipment at what moment it should begin transmitting its signals to the eNB (e.g., this can be expressed as a timing offset from a system reference timing). The timing advance command may be determined by the eNode B, e.g., by measuring the signals transmitted by the UE.
The eNode B transmit timing may drift over a period of time, e.g., due to temperature variation or due to the imperfections of the eNode B clock. Different UEs are usually distanced from the eNB by different amounts. With the propagation delay of a user equipment's signal to the eNB depending on the distance from the eNB to the user equipment, the UEs generally need to transmit their data at respectively different points in time in order for their transmitted signals to be synchronized with one another at the moment that they arrive at the eNode B receiver. In an effort to accomplish this, the UE automatically/autonomously adjusts its transmit timing using the timing advance command and a downlink cell-specific reference signal (CRS) received from the eNode B. The CRS signal is transmitted by the eNode B every sub-frame, e.g., to assist the UE in acquiring synchronization, performing demodulation, performing neighbor cell measurements, etc. The UE may also use other signals for UL timing adjustment. An example of such other signals includes synchronization signals, (i.e., a primary synchronization sequence (PSS)/a secondary synchronization sequence (SSS) in LTE).
Transmitting uplink signals in LTE using single carrier frequency division multiple access (SC-FDMA) technology requires that UL signals from all UEs received at the eNode B be orthogonal. The timing advance command in LTE ensures that the signals from all UEs in a cell are almost simultaneously received at the serving eNode B at about the same time regardless of their propagation delay thereby ensuring high degree of UL orthogonality. This is of increased importance in medium and large cells where there can be a large variation in the propagation delay of UEs in a cell.
In contrast to LTE, there is no Node B controlled timing advance command for adjusting the UE transmit timing in WCDMA. Instead, the UE automatically adjusts its transmit timing, e.g., if the Node B's DL transmit timing drifts, according to 3GPP TS 25.133 (see section 7.1.2) [1]. When the transmission timing error between the UE and the reference cell exceeds 1.5 chips, the UE must adjust its UL timing to within ±1.5 chips. The Node B needs to maintain the DL transmit timing of active cells within ±148 chips according to 3GPP TS 25.133 (see section 7.2.2). A UE must support reception, demodulation, and combining of signals of a downlink DPCH, or a downlink F-DPCH, when the receive timing is within a time window of T0±148 chips before the transmit timing, T0=1024 chips. In WCDMA, the uplink signals from different UEs may be received at different times. Hence, UL transmissions are not orthogonal in WCDMA.
Thus, one challenge with multi-RAT carrier aggregation is that different RATs on different carriers may have different timing and/or orthogonality requirements and different timing approaches.
For a multi-RAT carrier aggregation capable UE (e.g., HSPA+LTE), one approach to handle UE timing is for a UE to transmit an LTE carrier using LTE-specific rules, e.g., using a timing advance command to make automatic timing adjustments, and for the UE to transmit a WCDMA carrier using WCDMA-specific rules where the UE makes its own timing adjustments, e.g., without a timing advance command. But with these different timing adjustment approaches, the uplink transmit timings for the LTE and WCDMA carriers may drift apart. As a result, the Node B and eNode B may not be able to maintain the timing alignment between the DL carriers in the different RATs (e.g., HSPA and LTE). This means the UE may receive physical channels (e.g., HS-PDSCH and PDSCH in HSPA and LTE, respectively) containing data or signaling at different times so that there is a relative delay=Δτ between them. As a result, the UE cannot process the physical channels at substantially the same time. But as mentioned above, feedback signals from the UE, (such as ACK/NACK signaling), for both (or multiple) physical channels are sent over the same uplink control channel, (e.g., in the HS-DPCCH), within a certain offset time. Hence, a larger relative delay Δτ may lead to a reduced processing budget for HSPA or LTE carriers. A reduced processing budget leads to increased UE complexity, additional memory for storage, etc. It also leads to increased base station complexity (e.g., buffer size) in a RAN multi-RAT aggregation case, due to a possible longer packet reordering of a same user (on RLC layer or MAC layer) from LTE and HSPA with different timings. For a core network multi-RAT aggregation case, TCP time-outs could increase due to unsynchronized packets from two different radio systems.
Commonly-assigned U.S. patent application Ser. No. 12/869,693, filed on Aug. 26, 2010, entitled “Timing of Uplink Transmissions in a Multi-carrier Communications System,” proposes that a multi-carrier capable UE select a reference downlink carrier and use the reference downlink carrier's timing to determine a transmission time period based on an offset specified by a timing advance (TA) command. The transmission time period comprises a start time and a stop time. The Ser. No. 12/869,693 application is UE-based, and there is no coordination between multiple RAT accesses on a radio network level. Consequently, for multi-RAT communication, the two or more radio access networks are not aware if the UE is using one timing advance command for all of the radio access networks. From a network performance perspective, it is desirable for all the UEs to have the same timing rule so that uplink orthogonality can be maintained. Therefore, the timing requirements and signaling should be pre-defined.
Another issue is that a reference downlink carrier and associated time advance command may not always be available. For example, in a scenario where a UE uses the HSPA as a master system, when there is a need or desire to start using another RAT, like LTE, so that more data can be transmitted, the Ser. No. 12/869,693 application does not describe determining or signaling a timing advance, which is necessary in LTE. Furthermore, for a multi-carrier or a multi-RAT system where more than one timing advance is available (e.g. TD-LTE, TD-SCDMA, GSM), the Ser. No. 12/869,693 application does not disclose determining a common timing advance from a network perspective for use by the UE in a multi-RAT carrier aggregation system.